complement

from Dictionary.com/Word of the Day:

complement \KOM-pluh-muhnt\, noun:
1. Something that fills up or completes.
2. The quantity or number required to make up a whole or to make something complete.
3. One of two parts that complete a whole or mutually complete each other; a counterpart.

transitive verb:
To supply what is lacking; to serve as a complement to; to supplement.

Complement is from Latin complementum, from complere, "to fill up," from com- (intensive prefix) + plere, "to fill."

Usage note: Complement and compliment ("an expression of admiration or praise") are sometimes confused because they are pronounced the same. A good way to remember which is which is to make a connection between the spelling of complement and complete.

I take exception to the notion complement and compliment are pronounced the same. The middle syllable of the first word contains a short e sound (pleh) while the middle syllable in the latter word contains a short i sound (plih). When said correctly, there is a slight difference. Unfortunately, as with many things, we've now become lazy speakers so the difference that should be there really isn't anymore. Hence the 'pluh' pronunciation of the middle syllable.